Added a hook in validate.php for diagnostic assessment redirects. (#273)

This allows for defining a custom function to determine if a
user should be redirected to a diagnostic assessment.

Author: kittysnacks

hooks.md

@@ -133,6 +133,9 @@ Called when checking if LTI user can access the requested page
 return an array of base filenames that are allowed to be accessed by a user
 accessing an assessment via LTI.  This is merged with the default list.
 
+Called to determine if a user should be redirected to a diagnostic assessment
+`isDiagnostic()`
+
 ## util/batchcreateinstr
 
 Include via `$CFG['hooks']['util/batchcreateinstr']`

validate.php

@@ -465,7 +465,12 @@
     if (isset($_GET['graphdisp'])) {
         $_SESSION['graphdisp'] = $_GET['graphdisp'];
     }
-    if (isset($_SESSION['isdiag'])) { // && strpos(basename($_SERVER['PHP_SELF']),'showtest.php')===false) {
+    if (!function_exists('isDiagnostic')) {
+        function isDiagnostic() {
+            return isset($_SESSION['isdiag']); // && strpos(basename($_SERVER['PHP_SELF']),'showtest.php')===false) {
+        }
+    }
+    if (isDiagnostic()) {
         $urlparts = parse_url($_SERVER['PHP_SELF']);
         if ($_SESSION['diag_aver'][0] == 1 &&
             !in_array(basename($urlparts['path']), array('showtest.php', 'ltiuserprefs.php'))